Abstract The corrosion behavior of aluminum and three aluminum–silicon alloys in different concentrations of HCl solutions and its inhibition by antihypertensive drugs was studied using potentiostatic polarization measurements. As the acid concentration increases, the rate of corrosion increases. Aluminum is less susceptible to corrosion than any of Al–Si alloys. The inhibition efﬁciency of the drug compounds increases with their concentration up to a critical value. At higher additive concentrations the inhibition efﬁciency starts to decrease. The inhibitive action of these compounds is due to their formation of insoluble complex adsorbed on the metal surface. The adsorption follows Langmuir adsorption isotherms. It was found that the drugs compounds provide protection to Al and Al–Si alloys against pitting corrosion by shifting the pitting potential to more positive direction until critical drug concentrations (250 ppm). After this critical concentration the inhibition against to pitting corrosion starts to decrease.
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1. Introduction Aluminum and its alloys exhibit corrosion resistance in many environments and for this reason they ﬁnd many important industrial applications. The corrosion resistance is due to the initial formation of a compact and adherent passive oxide ﬁlm on the exposed surfaces. However, in the presence of insidious ions such as chloride ions, the protective oxide ﬁlm can be locally destroyed, initiating metal dissolution. Again, the oxide ﬁlm is amphoteric and hence dissolves readily in acidic solutions (Oguzie, 2009). In an attempt to mitigate electrochemical corrosion of aluminum and its alloys, the main strategy is to effectively isolate the metal from corrosion agents. This can be achieved by

226 the use of corrosion inhibitors. The use of inhibitors is one of the best known methods of corrosion protection. Most of the efﬁcient acid inhibitors are organic compounds that contain mainly nitrogen, sulphur or oxygen atoms in their structure. Organic compounds used as inhibitors act through a process of surface adsorption, So the efﬁciency of an inhibitor depends not only on the characteristics of the environment in which its acts, the nature of the metal surface and electrochemical potential at the interface but also on the structure of the inhibitor itself, which includes the number of adsorption active centers in the molecule, their charge density, the molecular size, the mode of adsorption, the formation of metallic complexes and the projected area of the inhibitor on the metal surface (Chetouani et al., 2005; Okafor and Zheng, 2009). Compounds with functional groups containing heteroatoms which can donate lone pairs of electrons are found to be particularly useful as inhibitors for aluminum corrosion (Khaled and Al-Qahtani, 2009; Obot and Obi-Egbedi, 2008; Lashkari and Arshadi, 2004; Zheludkevich, 2005; Maayta and Al-Rawashdeh, 2004; Amin et al., 2005; Obot et al., 2009). Also, compounds with p-bonds also generally exhibit good inhibitive properties by providing electrons to interact with the metal surface (Yildirim and Cetin, 2008; Hasanov et al., 2007; Umoren and Ebenso, 2008). Both features obviously can be combined within the same molecule such as drugs. The use of drugs as corrosion inhibitors for metals in different aggressive environments is not widely reported. Few reports exist in the literature to date; these include the use of sulpha drug (El-Naggar, 2007). Some pharmaceutical com-

M. Abdallah et al. pounds used for inhibition of Al in 0.5 mol LÀ1 H3PO4 (Fouda et al., 2009). In the previous work, rhodanine azo sulpha drugs (Abdallah, 2002) and antibacterial drug (Abdallah, 2004) were used as corrosion inhibitors for corrosion of 304 SS and aluminum in hydrochloric solutions. They inhibit the corrosion by parallel adsorption on the surface of the metal due to the presence of more than one active center in the inhibitor molecule. The aim of the present paper is to study the inhibiting action of some antihypertensive drugs on the general and pitting corrosion of Al and three alloys of Al–Si in hydrochloric acid solution using potentiostatic and potentiodynamic anodic polarization techniques. The mode of adsorption and the corrosion inhibition mechanism are also discussed. 2. Experimental methods The chemical composition of the three of Al–Si alloys is presented in Table 1. These electrodes in the form of a cylindrical rod were ﬁxed to pyrex glass tubing by araldite (exposed surface area is 0.79 cm2 for Al pure, 0.64 cm2 for alloy I, 0.77 cm2 for alloy II and 0.65 cm2 for alloy III). Electrical contacts were made through thick copper wires soldered to the end of the electrodes not exposed to the solution. The electrodes were successively abraded with different grades of emery paper, degreased with acetone and ﬁnally washed twice with distilled water; complete wetting of the surface was taken as an indication of its cleanliness. All chemicals used were of A.R. quality. The

Antihypertensive drugs as an inhibitors for corrosion of aluminum and aluminum silicon alloys in aqueous solutions solutions were prepared using twice distilled water and no trial was made to deaerate them. The electrochemical cell was all Pyrex and described elsewhere (El-Etre, 2007). The experiments were carried out at 25 ± 1 °C using air thermostat. Potentiostatic and potentiodynamic anodic polarization measurements were carried out using PS remote potentiostat with PS6 software for calculation of some corrosion parameters e.g., corrosion current density (icorr.) corrosion potential (Ecorr.) and rate of corrosion (Rcorr.). The corrosion parameters were calculated from the intercept of the anodic and cathodic Tafel lines. The potentiostatic and potentiodynamic anodic polarization measurements were carried out at scan rate of 10 mV sÀ1and 1 mV sÀ1, respectively. A three compartment cell with a saturated calomel reference electrode (SCE) and a platinum foil auxiliary electrode was used. The inhibition efﬁciency (I.E.) and the surface coverage (h) were calculated using the following equations: Rcorr:add I:E: ¼ 1 À 100 ð1Þ Rcorr:free Rcorr:add ð2Þ h¼ 1À Rcorr:free where, Rcorr.free. and Rcorr.add are the rate of corrosion in the absence and the presence of inhibitors, respectively. Conductance measurements were carried out using YSI model 32 conductance meter of cell constant equal to 1.6. The inhibitors used in this study were three compounds of antihypertensive drugs. The chemical structure of three compounds is shown in Table 2. 3. Results and discussion 3.1. Potentiostatic polarization 3.1.1. Effect of acid concentration Fig. 1 shows the anodic and cathodic polarization curves of Alloy III in different concentrations of HCl solutions as an exam-

227

ple. Similar curves were obtained for the other two alloys and Al pure (not shown) (see Table 3). The effect of acid concentrations on the corrosion parameters such as Ecorr., icorr. and Rcorr. is summarized in Table 1. Inspection of this table reveals that Ecorr. is dependent of acid concentration. Ecorr. values shifted more negative potentials. The values of icorr. of Al and all the alloys increased with increase of acid concentrations and consequently the corrosion rate Rcorr. increases. At the same acid concentration the value of icorr. decreases in the following order III > II > I > Al. This indicates that alloy III has the highest susceptibility to corrosion and Al is less susceptible to corrosion than any of these alloys in agreement with previous results (Bohni and Uhlig, 1969). 3.1.2. Effect of antihypertensive drugs The effect of addition of increasing concentration of three compounds of antihypertensive drugs (I–III) on the anodic and cathodic polarization curves of aluminum electrode and three of Al–Si alloys in 0.01 M HCl solution was studied. Similar curves to Fig. 1 were obtained (not shown). The corrosion parameters of Al and three Al–Si alloys such as Ecorr., icorr. and Rcorr. were calculated and listed in Tables 4–6, respectively. By the inspection of these tables, it is clear that the values of Ecorr. are changed by increasing the concentration of drugs. It is clear from Table 4 in presence of compound I. The values of Ecorr. are shifted to more positive potential. This indicates that this compound acted as an anodic inhibitor for Al, alloy I, alloy II and alloy III. In Table 5 compound II generally acted as cathodic inhibitors (except Al and alloy II, 1000 ppm compound II) On the other hand in Table 6 compound III generally acted cathodic inhibitor (except Al 1000 ppm compound III). When the additive concentrations increase from 50 up to 250 ppm the values of icorr. and hence Rcorr. decrease. Then at concentrations more than 250 ppm the values of icorr. and Rcorr. increase; consequently, the value of inhibition efﬁciency (IE) increases at concentrations up to 250 ppm. But at higher

Figure 1

Anodic and cathodic polarization curves of alloy III in different concentrations of HCl solution.

concentrations more than 250 ppm of drugs, the values of I.E. decrease. This indicates that the resistance to corrosion starts to decrease. At one and the same inhibitor concentrations the values of I.E. decrease according to the following sequence: Compound I > Compound II > Compound III: 3.1.3. Adsorption isotherm The adsorption of antihypertensive drugs on the surface of aluminum and its alloys can be interpreted by ﬁnding a suitable isotherm which describes the variation of experimentally obtained values of the amount of adsorbed substance by unit area of the metal surface with its concentration in bulk solution at constant temperature. The degree of surface coverage (h) which represents the part of metal surface covered by drug molecules was calculated using the following Eq. (2). The values of (h) for different concentrations of the studied drug compounds have been used to explain the best isotherm for adsorption of drug compounds on the metal surface. It is regarded as substitutional adsorption process between the drug compound in the aqueous phase (drug aq.) and water molecules adsorbed on the metal surface (H2O)ads (Moretti et al., 1999). Drugðsol:Þ þ XðH2 OÞads
DrugðadsÞ þ XðH2 OÞsol ð3Þ

isotherm. By far the results were best ﬁtted by Langmuir adsorption isotherm according to the following equation: C 1 ¼ þC h K ð4Þ

where K and C are the equilibrium constants of adsorption process and additive concentration, respectively. Plotting C/h against C (Fig. 2) gave a straight line with unit slope value with correlation coefﬁcient of 0.999, 0.996 and 0.998 for compounds I, II and III, respectively, indicating that the adsorption of antihypertensive drug on the surface of Al and Al–Si alloys follows Langmuir adsorption isotherm. From these results one can postulate that there is no interaction between the adsorbed species. 3.2. Potentiodynamic anodic polarization 3.2.1. Susceptibility of Al and its Alloys to pitting corrosion by chloride ions Fig. 3 represents the potentiodynamic anodic polarization curves of Al electrode in different concentrations of NaCl solution at scan rate of 1 mV sÀ1. Similar curves (not shown) were obtained for other three alloys. Inspection of the curves of this ﬁgure reveals that: (i) There is no any active dissolution oxidation peak was observed during the anodic scan. This reﬂects the stability of the air-formed oxide ﬁlm on surface of aluminum or its alloys.

where, X is the size ratio, that is, the number of water molecules replaced by one drug molecule. Attempts are made to ﬁt (h) values to various isotherms including, Frumkin, Temkin, Freundlich, Langmuir, Flory Huggins and Bockris-Swinkel

(ii) Increasing the sodium chloride concentrations causes the current ﬂowing along the passive region to increase suddenly and markedly at some deﬁnite potential denoting the destruction of the passivating oxide ﬁlm and the initiation of visible pits. The effect of increasing the chloride ions concentrations is the shift of the pitting potential into the active (negative) direction. The dependence of Epitt. with the concentration of ClÀ ion is shown in Fig. 4. The relation presents sigmoid S-shaped curve that indicates a higher Epitt. value at the lower chloride ion concentrations. In this case, the rate of passive ﬁlm formation prevails over that the ﬁlm breakdown, which is clear from the small change of Epitt. into the negative direction of potential. Thus, the metal surface may undergo a repassivation (Abdallah, 2004; Abd El-Haleem, 1979). However, at relatively higher ClÀ ion concentrations, Epitt varies with ClÀ ion concentration according to a straight-line relationship in the following forms (Abdallah and Al-Karanee, 2009): Epitt: ¼ a1 À b1 log CÀ Cl ð5Þ

200 C/θ 150 100 50

0 0 50 100 150 C,ppm 200 250 300

Figure 2

Langmuir adsorption isotherm for alloy III.

where a1 and b1 are constants depending on both the nature and type of the aggressive anion and of the electrode. This is due to the destruction of the passive ﬁlm formed on the metal surface and the pits propagate without allowing to undergo the repassivation. At higher ClÀ ion concentrations, Epitt shifted rapidly to more negative potential; it can be expected that

Antihypertensive drugs as an inhibitors for corrosion of aluminum and aluminum silicon alloys in aqueous solutions

231

Figure 3

Potentiodynamic anodic polarization curves of pure Al in different concentrations of NaCl solutions.

-300
Al pure

-350 -400 -450

Alloy I Alloy II Alloy III

Epitt, mV (S.C.E.)

-500 -550 -600 -650 -700 -750 -800 -4 -3 -2 log C,M -1

Figure 4

The relationship between pitting potential and logarithm of the concentrations of NaCl solution.

the ﬁlm breakdown exceeds the ﬁlm formation and the pit is continuously propagated. Further inspection of the curves of Fig. 4 reveals that at one and the same ClÀ ion concentration the shift of pitting potential (Epitt) to active (negative) values decreases in the following order: Al > Alloy I > Alloy II > Alloy III

This sequence differs from that sequence obtained by general corrosion of Al and its alloys, where Al is more resistant to corrosion in HCl solution than Al–Si alloys. In general corrosion the oxide ﬁlm formed by Al is thick, adherent and non porous. The addition of Si as an alloying element increases the rate of corrosion. Since Al is trivalent and Si is tetravalent, the excess of electron d delocalized throughout the lattice producing point defect. The point defect increases with

232 increasing Si content. This led to the decrease of the resistance of Al. In the localized attack (pitting) by chloride ions, the presence of Si as an alloying element increases the pitting corrosion resistance of Al (Abd El-Rehim et al., 2004). This can be attributed to the incorporation of Si atoms in the passive ﬁlm (Mzhar et al., 2001). This incorporation repairs the ﬁlm defect and renders it more stable (Strehblow and Doherty, 1978). The above sequence may be attributed to that the chloride ions attack the passive ﬁlm which contains mainly Al2O3 and SiO2. The increase of Si content in the alloy led to form solid solutions consequently increases the resistance to pitting attack. This sequence shows that pure Al has the highest susceptibility to pitting corrosion and alloy III is the less susceptible to pitting corrosion than any of other two alloys. 3.2.2. Inhibition of pitting corrosion The effect of increasing addition of the studied drug compounds on the potentiodynamic anodic polarization curves of Al and its alloys in 1 · 10À2 M NaCl solution was studied. Similar curves (not shown) to those of Fig. 3 were obtained in the presence of these compounds. In their presence, the pitting potential was shifted toward a more positive direction until concentration of drug was up to 250 ppm. This indicates that inhibitive effect of these compounds for pitting corrosion. At higher concentrations (more than 250 ppm) of drugs, there is a shift of Epitt., into the active (negative) direction. This shift indicates that the resistance to pitting corrosion is decreased. Fig. 5 represents the relationship between Epitt and log Cinh. From the curves of this ﬁgure, it is clear that the increase of the concentration of the antihypertensive drugs until a critical concentration (250 ppm) causes a shift of pitting corrosion potential into the noble (positive) direction, in accordance with the following equation:
-300
Compund I Compound II

M. Abdallah et al. Epitt: ¼ a2 þ b2 log Cinh: ð6Þ

where a2 and b2 are constants depending on the type of inhibitors and aggressive anions as well as the metal or alloys under test. This denotes that at this critical concentration these compounds start to lose their inhibiting effect. Some authors (Shams El-Din et al., 1977; Abd El Haleem et al., 1995) attributed the above observation to the hydrolysis of inhibitor to produce corrosion promoting species. However, one may attribute the lose of inhibiting effect toward pitting aluminum and its alloys in presence of high concentrations of antihypertensive sites drugs (more than 250 ppm) due to the competition for adsorption sites on the metal surface. The accumulation of the inhibitors molecules on the metal surface which creates a steric hindrance effect (Schweinsberg et al., 1988). Such effect leads to loosely attack the layer which stimulates corrosion rather than inhibition. 4. Mechanism of inhibition The inhibition of the general and the pitting corrosion of pure aluminum and aluminum silicon alloys in hydrochloric acid solutions by some antihypertensive drugs as measured by potentiostatic polarization and potentiodynamic anodic polarization were found to depend on both the concentration and the nature of the inhibitors. As the concentration of the inhibitors increases the observed corrosion parameters led to: (i) (ii) (iii) (iv) Decrease of corrosion density. Increase of inhibition efﬁciency. Increase of surface coverage. Shift of pitting potential to positive direction.

-350

Compound III

-400

-450

-500

-550

-600 1.5

1.7

1.9

2.1

2.3 2.5 log C,ppm

2.7

2.9

3.1

Figure 5 The relationship between pitting potential and logarithm of the concentrations of inhibitor compounds in 1 · 102 M NaCl solution for alloy III.

The inhibition efﬁciency of antihypertensive drugs against the corrosion of Al and Al–Si alloys in 1 · 10À2 M HCl was explained on the basis of the adsorption of the inhibitors at the electrode-solution interface (Mohmoud et al., 1996). Since the drug compounds contain more than one active center in their chemical structures, they will improve the adsorption process, and consequently inhibit the metals against corrosion. However, the inhibition efﬁciency of the studied compounds depends on many factors, which include the number of adsorption active centers in the molecule, charge density, molecular size, structure and mode of interaction with metal surface and ability to form complexes (Fouda et al., 1986). To illustrate the mechanism of interaction of antihypertensive drugs with metal ions, the stoichiometry of the expected Al-drugs complexes was estimated by conductance measurements. Conductometric titration curves were obtained by titrating 50 ml of 1 · 10À3 M Al3+ with a solution of 1 · 10À4 M drugs compound as a titrant. The conductance ml-added curves (Fig. 6) are characterized by breaks at molar ratio of 1.0 metal cation: 1.0 drugs additives for compounds I, II and III (cationligand). It is known that (Amin, 1995) the shape of the conductometric curve depends on the concentrations of all the species present during the titration process as well as on some other factors such as viscosity, dielectric constant, solvation, complexation and proton transfer. The inhibition process of antihypertensive drugs can be attributed to the formation of insoluble complexes. The three

nation bonds (O ﬁ Al). On the other hand, structure III contains two covalent bonds. Since the covalent bond (O-Al) is stronger than the coordination bond (O ﬁ Al), thus, structure III is the most expected complex to be formed. Compound II interacts with the metal ion to give the following structure.

ΩCorr.

As seen, this structure contains two bonds; one of them is coordination (N ﬁ Al) and the other is covalent bond (O–Al). Compound III reacts with the metal ions and the following structure may be obtained.

compounds of antihypertensive drugs can react with Al3+ ion according to the following reaction: Compound I (high molecular weight) which gives the highest inhibition efﬁciency, can react with Al3+ ion via one of three routes to give structures I, II or III. Structure (I)

The last structure contains two bonds; one of them is coordination bond (N ﬁ Al), while the other bond (O–Al) is covalent. In view of the above observations, one can conclude that the inhibition efﬁciencies (I.Es) of antihypertensive drugs (I– III) decrease in the order: Compound I > Compound II > Compound III: This sequence is in a good agreement with the results obtained experimentally from the two techniques.

5. Conclusion (1) Antihypertensive drugs act as inhibitors for general and pitting corrosion of Al and Al–Si alloys. (2) The inhibition efﬁciency increases with increasing drug concentrations up to a critical value and starts to decrease in presence of higher additives’ concentrations due to steric hindrance effect. (3) The drug compound acts as corrosion inhibitor due to the formation of insoluble complex adsorbed on the metal surface. (4) The adsorption process follows Langmuir adsorption isotherm. (5) Al–Si alloys are more resistant to pitting corrosion than Al in chloride-containing solution.